I don't think there is a way to control the loop parameters except maybe some PFORTH commands. The Trimble units can be put into holdover via software. The Symmetricom units require you to disconnect the antenna (or possibly use a SCPI command to manually set the DAC voltage... Lady Heather does this with the Symmetricoms).
Trimble's save the survey in EEPROM, Symmetricom's do not. You can manually enter a position and stop the survey.
I got bare boards (one for $40!) since I bought them just to get Lady Heather working with them. If you want to pay $30 - $60 extra to get one in a box is up to you and the bulgeosity of your wallet multiplied by your coefficient of lazy bastardness and divided by the Q factor of your box stash.
I have not done any ADEV measurements on them, but here a couple of links to EEVBLOG users that have. It looks like they could use a little tuning. One guy claims the Symmetricom oscillators have around 10 dB better phase noise (around 3dB worse than a HP-10811) than the Trimbles .... YMMV in the oscillator phase noise sweepstakes.
http://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/?action=dlattach;attach=227310;image
http://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/?action=dlattach;attach=206950;image
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> How good is the OCXO? Can you adjust the loop tuning parameters for best
stability with the individual OCXO? If not, does the loop cross over
from the OCXO to the GPS at an appropriate tau, or does it exhibit the
notorious "crossed over too early" hump in the xDEV plots? Does it
store its surveyed location or does it need to do a new survey every
time it is powered up